1. Field of the Invention
This invention relates generally to gear arrangements for transmitting torque through an angle and, more particularly, to gear arrangements for transmitting torque from an engine shaft to a parallel offset output shaft.
2. Description of the Related Art
A variety of gear arrangements have been used for transmitting torque from an engine shaft to an offset driving shaft, which requires transmitting torque twice through an angle. That is, the torque must make two approximately 90.degree. turns such that the engine shaft and the driving shaft rotate about parallel but offset axes. The requirement of making two 90.degree. turns, or turning the corner twice, adds complexity to the gear arrangement. Conventionally, the offset is made using bevel gears or a train of spur or helical gears, often in conjunction with planetary gear stages for speed reduction.
Bevel gears comprise gear wheels with straight teeth that mesh at an angle. That is, the top edges of the gear teeth are inclined relative to the axis of rotation and define a cone. Spiral bevel gears are also commonly used to turn a corner, and have curved teeth that mesh at an angle. The ratio of teeth on the various gear wheels, resulting in differences in speed, is referred to as the speed reduction ratio. Spiral bevel gears can transfer a great amount of torque for their size and are used, for example, in a typical automobile differential to transmit torque from a drive shaft to a rear axle. Spiral bevel gears are also used in conventional helicopter transmissions, where rotor blades must rotate about an axis that is approximately perpendicular to the helicopter engine shaft. While spiral bevel gears are especially suited for transmitting large amounts of torque through angles, they do have drawbacks.
Spiral bevel gears are extremely sensitive to changes in the relative location of the gears. In fact, spiral bevel gears are often located to a tolerance of thousandths of an inch. Thus, spiral bevel gears are adversely effected by small amounts of thermal growth during operation and by deflections of the gear supporting structure under load. Therefore, the gears may not maintain an optimum mesh through changes in temperature and deflections of the supporting structure. This sensitivity limits the speed reduction ratios for which spiral bevel gears can be advantageously employed. Such sensitivity also means that spiral bevel gears require complex manufacturing, assembly, and gear mating procedures, and consequently are more expensive to employ than simpler gear arrangements. The sensitivity to minute changes in relative position also requires a heavy and complex support structure for spiral bevel gears.
Because maintaining optimum mesh is so critical to high speed operation, spiral bevel gears are more readily suited to low speed reduction ratios (ratios less than two-to-one) and are less suited to high speed reduction ratios (greater than four-to-one). It can easily be necessary to use a complicated gear arrangement to achieve overall gear reduction ratios of ten-to-one or more. For example, the required overall speed reduction is often obtained using a combination of bevel gears for turning the corner and also planetary gears for speed reduction.
Gear arrangements using a spur gear pinion/face gear combination for turning a corner are much more forgiving of changes in relative size and position than spiral bevel gears. A spur gear is a gear wheel with straight radial teeth on its circumference, the teeth edges being parallel to the axis of rotation and defining a cylinder. A face gear is a gear wheel having an axis of rotation perpendicular to that of the spur gear and having teeth on its face along the wheel periphery, the teeth being radially directed toward the center of the wheel. Alternatively, the spur gear and face gear can have helical teeth, in which the teeth edges are straight but are inclined from the axial and radial direction, respectively. In either case, the top edges of the face gear teeth define a plane rather than the spiral bevel gear cone.
A spur gear can move in and out freely along its axis of rotation, within the limits of the length of its teeth, without negative effects on meshing with the face gear. Furthermore, a spur gear has more tolerance for movement toward or away from its driven face gear than does an equivalent spiral bevel gear arrangement. Thus, gear arrangements using a spur gear and face gear to turn a corner are relatively unaffected by the thermal growth and deflection of the supporting structure experienced in many applications. For this reason, such gear arrangements easily accommodate speed reduction ratios greater than four-to-one.
Spur/face gear combinations exhibit true conjugate action in the presence of small deflections and misalignments. Therefore, for a spur/face gear combination, the velocity of the driven gear remains constant as the teeth go through meshing engagement, resulting in relatively quiet and vibration free operation. Spiral bevel gears do not exhibit conjugate action in the presence of equivalent deflections and misalignments. Therefore the velocity of the driven gear experiences minute acceleration and deceleration as the teeth go through meshing engagement, and manifests itself as objectionable noise and vibration.
A spiral bevel gear stage can be used to transfer the torque around the corners, ordinarily with less than a two-to-one speed reduction ratio, and a spur gear and/or planetary gear stage can be used for the remaining speed reduction to achieve the overall ratio desired. Unfortunately, such gear arrangements can be rather complex, requiring many gear stages and supporting bearings. Such planetary gear stages where an aircraft engine turns propeller blades, for example, can have as many as ten gears and twelve to fifteen bearings. Each one of these gear stages and bearings presents a possible failure mode for the system, decreasing the reliability while increasing the complexity, cost, weight, and associated problems.
Thus, there is a need for a compact gear arrangement that can transmit large amounts of torque through an offset path while accommodating thermal growth and deflection of the supporting structure, with reduced vibration and noise, capable of operating at large speed reduction ratios, while minimizing the number of gear stages and supporting structure for reduced weight and greater reliability. The present invention satisfies this need.